When a shaft is provided on a metallic plate, it has been a usual practice to fix a shaft prepared separately on a metallic plate by caulking, spot welding and the like.
An explanation will be made below on the prior art methods for forming a shaft on a metallic plate.
FIG. 17 shows a prior art method for forming a shaft on a metallic plate by caulking.
A hole is formed on a metallic plate 10, on which a shaft 11 is to be attached, and the shaft 11 prepared separately is fixed on the metallic plate 10 by caulking.
FIG. 18 shows another prior art method for forming a shaft on a metallic plate by spot welding.
A shaft 13 prepared according to a separate process is fixed by spot welding on a metallic plate 12, on which the shaft 13 is to be mounted.
The Japanese patent gazette Heisei 6(1994)-26737 describes a method for forming an shaft as shown in FIG. 19, whereby, after a hole has been formed on one surface of a plate material 21 and a projection 24 on the other surface thereof by pressing forcefully a first tool 22 against the surface of the plate material 21 a bottom surface 23a of a second tool 23 is pressed forcefully against the foregoing hole, thereby a certain amount of material being pushed out to make the projection 24 further protrude and then the first tool 22 is again pressed against the hole to increase the height of the projection 24, thus completing the formation of a shaft.
In addition, it is known that there are a forward extrusion method as shown in FIG. 20 and a backward extrusion method as shown in FIG. 21 as the methods for making things in an axial shape.
However, the prior art method for forming shafts by caulking or spot welding as described in the above comprises the steps of preparing shafts separately and then fixing the shafts on a metallic plate with a resulting increase in cost.
Furthermore, in the case of caulking, the metallic plate involved is liable to be deformed due to the pressure applied by caulking and also it is rather difficult to achieve squareness between the metallic plate and the shaft that are to be put together, and in the case of spot welding, sufficient strength is unable to be gained due to a relatively small welding area, thus presenting rather a serious problem.
In the case of the method as described in the Japanese patent gazette Heisei 6(1994)-26737, since a pushing down force of tool is always applied to the boundary between a plate material and a projection, it is difficult to obtain a sufficient height of the projection and also it is difficult for the projection to achieve squareness against the plate material and accuracy in the outer diameter thereof with an additional problem of requiring many steps in processing.
Also, both the forward extrusion method and backward extrusion method are the methods intended for producing things in an axial shape from a material in pellet form and not for forming integrally a shaft by extruding from a flat plate.